The present invention relates to a liquid drop discharge device for printing by discharging a discharge liquid (ink).
In an ink jet discharge device for printing by discharging liquid drops through nozzles, a print head and an ink tank are ordinarily connected by a dedicated use flow path, so that the ink stored inside the ink tank is fed to the print head through this flow path.
The fed ink is pushed out as ink drops through a nozzle hole with a pressure wave generated by a pressure generating device (for instance, in an on-demand type ink jet head, an actuator (such as, a heater, a piezoelectric element or the like)) placed inside the print head.
At this time, in order to efficiently discharge the ink drops through the nozzle hole, the meniscus of the ink (the surface state of the ink) at a portion of the nozzle hole needs to be stably held when the print head is in no operation. In order to maintain the meniscus, a force that counters a force by which the ink drop spontaneously falls under gravitation must be given to the ink.
However, the above-mentioned on-demand type print head is equipped with a liquid drop ejecting mechanism, it is never provided with a mechanism for preventing the ink from leaking through the print head when the print head is in no operation. Therefore, a method in which a pressure (back pressure) is applied in order to prevent the leakage of the ink is used.
However, the minimum control range of the back pressure required in the ink jet discharge device is around about 10 mm H2O or less; and thus, a difference in pressure between an outside atmosphere and the back pressure is extremely small. For this reason, it is difficult in a system using a conventional vacuum pump or the like to control this pressure range with good precision.
As one of the conventional art techniques for providing the back pressure, there are numerous examples in which a network porous body is provided inside an ink tank (See JP Patent No. 2683187, JP Patent No. 3163864, JP Patent No. 3513979 and JPA 2007-62189).
According to this technique, a capillary force, which is generated by fine pores of the porous body when the ink is sucked into the porous body, is utilized as the back pressure to hold the ink. It is possible to control the capillary force based on dimensions, the material, the shapes of the fine pores, etc. In this method, however, there is a problem in that a component of the ink is adsorbed on the porous body because the back pressure control accuracy is low and a large amount of the ink is contained in the porous body.
As another method to provide the back pressure, there is an example in which such a porous body as mentioned above is not used (See, JPA 2006-123562). According to this reference, a movable lid, which is movable up and down, is provided at an interface portion between a gas and the liquid of the stored ink, and the ink is directly held by generating a negative pressure at this lid portion. However, in this method, a separate spring mechanism, which is to push up the movable lid, is necessary so as to hold the negative pressure, which may pose a limitation on the structure of the device.